It is widely known to locate and detect fingerprints deposited on metal or other surfaces using chemical reactions with either the eccrine (amino acid) and/or sebaceous (fatty acid) content of a fingerprint residue. Other methods of detecting fingerprints involve solvent techniques, e.g. cyanoacrylate fuming, or using the fingerprint residue as an insulator against electrochemical or reduction/oxidation (redox) reactions.
Such methods require the continued presence of the fingerprint residue and are not suitable for the visualisation of latent fingerprints.
Another known technique for detecting residual fingerprints measures the difference in electrical potential between a metal substrate on which a fingerprint has been deposited and a metal probe not in contact with the surface. The technique exploits a discovery made by Lord Kelvin in the nineteenth century, which is that different metals, connected electrically, have a potential difference between them determined by the work function of the metals. The work function of a particular metal is a measure of the ease with which an electron can leave the surface of the metal. The chemicals found in a fingerprint deposit cause corrosion of a metallic surface. The work function of a metal changes where it has been corroded by a fingerprint. Therefore a work-function based technique relies on measuring the differences in work function across the whole of a surface of a metal where it is suspected a fingerprint has been deposited. Clearly, this can be a time consuming and haphazard process.
The effects of increased temperature on a substrate are known to inhibit subsequent fingerprint detection. This is in part due to the eccrine and sebaceous content of the residue becoming evaporated and/or vapourised at high temperature.
In addition, a surface can become wet, e.g. from lying for sometime on the floor during wet weather, which may result in the residual fingerprint being washed from a substrate. Moreover, the perpetrator of a crime may have gone to his best efforts to disguise his identity by wiping or washing away his fingerprint from the substrate.
One particular problem has been trying to detect and develop fingerprints on small objects with tightly curved surfaces, e.g. brass shell casings. Due to the tight curvature of the surface, even when handled, a whole fingerprint is rarely deposited.
It is desirable to be able to locate and identify whole or partial fingerprints deposited on a wide variety of surfaces even when such surfaces have been exposed to extreme or outdoor conditions or have been cleaned, for example by persons trying to hide their actions.